Total dissolved solids (TDS) is a measure of the combined content of all inorganic and organic substances contained in a liquid. These inorganic and organic substances are normally referenced for freshwater systems.

You can use the HM digital® TDS meter to determin if your Reverse Osmosis membrane is removing impurities from your water. The DMW item number for the TDS meter is TDS-3: Handheld TDS Meter with Carrying Case

Follow the steps below to have our online calculator calculate your effectiveness of membrane.

Rinse the TDS Pocket Monitor with reverse osmosis water.

Test your Tap Water first with the TDS Pocket Monitor and key in the results below


Tap Water Result: 

Rinse the TDS Pocket Monitor with RO water again. Then test your RO water with your TDS monitor. Key in your results below

RO Water Result:  

Click on the calculate button below to view your results.

        Your Result is %,  this is your membrane effectiveness.

The rejection rate is the percentage of the amount of TDS (Total Dissolved Solids) eliminated from the tap water by the membrane. Example: A 90% rejection rate means that 90% of the TDS has been eliminated from the tap water.

If your Membrane Effectiveness is between 75-100% your membrane is operating within factory specifications.

If your Membrane Effectiveness is 74% or less then we recommend replacing your membrane and the flow restrictor as well as the pre and post filters.

Click here to go back to Total Dissolved Solids page